1. Field of the Invention
The present invention relates generally to medical stimulation devices, and more particularly, to focused stimulation in a medical stimulation device.
2. Related Art
There are several types of medical devices that use electrical signals to activate nerve, muscle or other tissue fibers in a recipient (also referred to herein as a user, listener, patient, etc.; “recipient” herein) to stimulate an activity. Such medical devices are generally referred to herein as medical stimulation devices. Usually the activity is invoked to compensate for a deficiency in the recipient. For example, stimulating prosthetic hearing devices have been developed to compensate for loss of hearing.
Several types of prosthetic hearing devices provide electrical stimulation to aid recipients who have a hearing deficiency. For example, cochlear™ implants (also referred to as cochlear™ devices, cochlear™ prosthetic devices, cochlear™ implants, and the like; simply “cochlear implants” herein) apply one or more stimulating signals to the cochlea of a recipient to stimulate hearing. Another example is an auditory brain implant that delivers electrical stimulation to the auditory brainstem nuclei of a recipient to stimulate hearing. For ease of description, the present invention is presented in the context of a prosthetic hearing device, namely, a cochlear implant. However, it should be appreciated that unless stated otherwise, the present invention is applicable to any medical stimulation device now or later developed.
Cochlear implants typically include a sound transducer that receives incoming sound, and a sound processor that converts selected portions of the incoming sound into corresponding stimulating signals based on an implemented sound encoding strategy. The sound processor transmits the stimulating signals along an electrode array implanted within or adjacent to the cochlea of the recipient.
Cochlear implants exploit the tonotopic organization of the cochlea by mapping audio energy in specific frequency bands to deliver stimulation at corresponding locations along the spiral array of auditory nerve fibers. To achieve this, the processing channels of the sound processor; that is, specific frequency bands with their associated signal processing paths, are mapped to a set of one or more electrodes to stimulate a desired nerve fiber or nerve region of the cochlear. Such sets of one or more electrodes are referred to herein as “electrode channels” or, more simply, “channels.”
Conventional cochlear implants have limitations that may produce undesirable effects for recipients. One fundamental problem that limits the spatial resolution of multi-channel cochlear implants is referred to as “current spread” and is illustrated in FIG. 1. Although stimulation through one channel is intended to excite a single nerve region, in fact the actual locus of neural excitation can be broad and complex due the spread of current throughout the conducting fluids and tissues of the cochlea.
FIG. 1 is a graph illustrating the voltage created at various electrodes in response to the application of current at one electrode. Voltage profile 101 illustrates the voltage created at different nerve regions of the cochlear (“tissue voltage”) at a plurality of locations adjacent electrodes 104 of an electrode array 106 in response to current delivered to electrode number 11. Superimposed on FIG. 1 is a illustration of the current spread 102 emanating from the nerve region adjacent electrode 11 which causes voltage profile 101.
As illustrated by voltage profile 101, current delivered by electrode 11 may spread over a potentially wide spatial extent of neighboring nerve regions. This current spread may extend, for example, to nerve regions adjacent to distant electrodes 1 and 22 of the 22 electrodes of electrode array 106. As a result, a stimulating voltage 101 arises not only in the nerve region adjacent electrode 11 but also at more distant nerve regions in the tissue. As shown in FIG. 1, the stimulating voltage 101 is strongest or most intense near electrode 11, dropping off slowly and, in this example, remaining non-negligible at all regions in the cochlea nerve adjacent to electrode array 106. As a result, in addition to the nerve fibers adjacent electrode 11, other nerve fibers in the cochlea are stimulated by current applied to electrode 11. This may produce a distributed place-pitch perception, rather than the single pitch percept that was intended by the stimulating method.
This problem is exacerbated when current flows concurrently from two or more electrodes, as would occur when representing a sound with multiple frequency components. When two or more channels are activated concurrently, the locus of excitation is not the simple union of their individual loci because of the nonlinearity of the neural excitation process. Instead, neurons that fall outside of the individual loci (i.e. those which would not respond to any one channel) may nevertheless be excited by the summed current fields. This results in the well-known phenomenon of “channel interaction” or “channel overlap.” Channel interaction can lead to unpredictable loudness fluctuations, and smearing of the spatial representation of spectrum.
FIG. 2 shows the consequences of current spread 2029 and 20213 when current concurrently flows from two electrodes 9 and 13 of electrode array 106. Voltage profile 208 is generated in response to stimulating electrode 9, while voltage profile 210 is generated in response to stimulating electrode 13. Voltage profile 212 is the sum of voltage profiles 208 and 210; that is, voltage profile 212 is generated in response to simultaneously stimulating electrodes 9 and 13. As shown in FIG. 2, the combined currents produce a stimulus voltage in the nerve region adjacent to each electrode which is greater than intended, as well as a high voltage 214 in the nerve region between the electrodes 9 and 13.
This summation of stimulus voltages has many undesirable perceptual consequences, particularly when many electrodes are activated simultaneously to represent a complex sound with multiple frequency components. For example, such stimulation may result in unpredictable and excessive loudness and loss of spectral shape, that is, the peaks of the frequency-place profile are distorted by the summation of fields.
Almost all successful stimulation strategies in clinical use today circumvent channel interaction by using sequential pulsatile stimulation. Such strategies deliver stimulation through only one channel at any given instant. Stimulation is time-multiplexed across channels at rates high enough to produce a fused percept for the recipient. Although monopoles excite broad spatial extents of the nerve array, spatial/spectral information is nevertheless adequately conveyed, presumably by the trajectory of the centroids of those ranges. In this way a reasonable representation of a sound's time-varying magnitude spectrum can be appreciated by the recipient, such that formant peaks can be perceived.
Prior to the widespread adoption of sequential-monopolar stimulation for clinical use, several more complex channel configurations were explored with the objective of producing more focused electrical fields and hence narrower stimulation regions. These included bipolar stimulation with longitudinally- radially-, and obliquely-oriented dipoles, tripoles or quadrupoles, common ground, and complex multipolar channels derived by so-called “current deconvolution.” These more complex channel configurations have not resulted in speech understanding gains.